1. Field of the Invention
The present invention relates to a backlight inverter for a thin film transistor-liquid crystal display (TFT-LCD) panel, and more particularly to a backlight inverter for an LCD panel which is capable of detecting a fault in a transformer device including two transformers driven in tandem or an open-lamp condition and performing a control operation to stop its operation upon detecting the fault in the transformer device or the open-lamp condition, so that an enhanced self-protection function can be carried out in the event of a malfunction, such as the fault in the transformer device or the open-lamp condition, to more securely prevent the internal components and circuits of the inverter from being damaged due to such a malfunction.
2. Description of the Related Art
In general, cold cathode fluorescent lamps (CCFLs) are operated at low current, resulting in advantages such as low power consumption, low heat, high brightness and long life. In this regard, the CCFLs have recently been used in various display devices such as a backlight unit of a computer monitor, for example, a TFT-LCD, and a display panel of a printer. A high alternating current (AC) voltage of 1 to 2 kV is required to light such a CCFL, and an inverter is utilized to provide such a high AC voltage.
The inverter can be generally classified into two types, a single type (or a single-stage type) where one transformer is driven by one driver and a double type (or a two-stage type) where two transformers are driven in tandem by one driver. In the case of the double-type inverter, a transformer device including the two transformers is adapted to operate two lamps in tandem with an AC voltage and a complementary AC voltage having a phase difference of 180 degrees therebetween, respectively.
FIG. 1 is a block diagram showing the construction of a general backlight inverter for an LCD panel. As shown in this drawing, the general backlight inverter comprises a switching device 11 for converting a direct current (DC) voltage of about 5 to 30V into a square-wave voltage in response to a pulse width modulation (PWM) signal, a rectifier 12 for rectifying an output voltage from the switching device 11 by half wave, a transformer driver 13 for converting an output voltage from the rectifier 12 into an AC voltage, a transformer device 14 for boosting an output AC voltage from the transformer driver 13 to a voltage level of about 1 to 2 kV necessary to a lamp operation, a lamp 15 connected to the transformer device 14 such that it is turned on/off in response to an output voltage from the transformer device 14, a feedback voltage detector 16 for detecting a voltage corresponding to current flowing through the lamp 15, and a dimming controller 17 for generating the PWM signal based on the voltage detected by the feedback voltage detector 16 and providing it to the switching device 11 to adjust a duty cycle of the square-wave voltage. The transformer driver 13 can be of any drive type based on a given circuit configuration.
This backlight inverter for the LCD panel is disadvantageous in that an over-voltage may be generated upstream of the transformer device or therein for various reasons when in use, causing damage to the internal components and circuits of the inverter. For example, in the case where the lamp is completely open, no current flows therethrough, so an over-voltage is generated upstream of the transformer device. Moreover, the lamp may be incompletely connected due to a poor connection of the peripheral components of the transformer device, resulting in a corona phenomenon where a spark discharge occurs due to an instantaneously high voltage in the transformer device itself. This corona phenomenon potentially involves the danger of fire.
In order to overcome the above problems, there have recently been proposed various backlight inverters for LCD panels, one example of which is disclosed in U.S. Pat. No. 6,259,615 and shown in FIG. 2 herein.
FIG. 2 is a circuit diagram showing the construction of a conventional backlight inverter for an LCD panel.
With reference to FIG. 2, the conventional backlight inverter comprises a transformer TX1, a power source 21, a bias/reference voltage generator 23 for generating a bias/reference voltage necessary to the internal operation of the inverter from the power source 21, and a switching device 28 for switching a voltage V1 from the power source 21 in response to a drive signal to provide a current path in the transformer TX1. To this end, the switching device 28 is composed of four transistors Switch_A to Switch_D. The conventional backlight inverter further comprises an LCD panel 22 including a CCFL operated by the transformer TX1, a protection circuit 26 for detecting an output voltage OVP from the transformer TX1 and providing a sweeping stop signal if the detected output voltage exceeds a reference voltage, and a frequency sweeper 27 for generating a 50% duty-cycle square-wave pulse by performing a frequency sweeping operation until the sweeping stop signal is provided from the protection circuit 26, namely, the output voltage OVP from the transformer TX1 exceeds the reference voltage. The conventional backlight inverter further comprises a feedback controller 24 for comparing a feedback voltage from the protection circuit 26 with a reference voltage and controlling an ON time of the switching device 28 in accordance with the compared result, and a driver circuit 25 for generating the drive signal in response to an ON-time control signal from the feedback controller 24 and the square-wave pulse from the frequency sweeper 27 and providing it to the switching device 28.
The protection circuit 26 includes a comparator 26A for comparing a voltage detected from the LCD panel 22 with a CMP voltage (reference voltage) and, if the detected voltage is above the CMP voltage, recognizing an open-lamp condition and providing the sweeping stop signal to the frequency sweeper 27, a timer 26B for initiating a time-out sequence once the detected voltage exceeds the reference voltage and allowing the comparator 26A to provide the stop signal once the time-out is reached, and a current sensor 26C for detecting current corresponding to the open-lamp condition to turn off the frequency sweeper 27.
The conventional backlight inverter for the LCD panel with the above-mentioned construction is desirable to protect the internal circuits from an over-voltage in the open-lamp condition, but has a disadvantage in that it cannot detect a fault in the transformer itself and, in particular, cannot propose any solution to a fault in a transformer device including two transformers driven in tandem, resulting from an internal or external poor connection of the transformer device.
In other words, in the case of the double-type inverter where two transformers are driven in tandem, a corona phenomenon may occur due to a poor connection of the peripheral components of the transformer device, which may cause damage to the inverter components and circuits. In a serious case, a strong spark in the transformer device may result in the occurrence of a fire.